Efflorescence, which is the presence of calcium carbonate or other relatively insoluble salts in powdery form on the surface of concrete or masonry products, is a familiar problem in the concrete industry. The salts are produced unintentionally and, whilst they do not generally lead to any significant failure, they affect the aesthetic quality of the product.
Efflorescence is most often found to be calcite (calcium carbonate) produced by a reaction between free calcium hydroxide in the product and atmospheric carbon dioxide. The free calcium hydroxide is produced by the normal hydration reactions of Portland cement, and tends to migrate to the surface of the product where the reaction with atmospheric carbon dioxide can occur. The timing of the appearance of efflorescence varies and can occur immediately following formation of the product or after the product has been installed. As an example, one product on which efflorescence can occur is a concrete roofing tile. The relatively minor whitening of cementitious products by other mechanisms is not included in the term “efflorescence”, as that term is used herein.
Depending upon the circumstances, efflorescence can be more or less difficult to remove. Some treatments, such as the use of an acid wash, are only temporary and the problem often reappears after a period of time.
Efflorescence can be reduced, but not eliminated, by the incorporation in the product of fine fillers which act to block the pores in the product, examples being fumed silica, metakaolin, limestone filler or polymers. An expensive method for preventing efflorescence is to coat the product with an impermeable polymer, but ensuring the impermeability of a thin polymer coating can be difficult.
Another method for reducing, but not totally eliminating, efflorescence in the product is to include in its composition a material rich in reactive silica. The calcium hydroxide reacts preferentially with the excess reactive silica, thereby preventing its reaction with atmospheric carbon dioxide.
The notation used herein is that commonly used by cement chemists, in which: C=CaO; S=SiO2; A=Al2O3; ŝ=SO3; and H=H2O.
We are aware of British patent GB 2099808B (Chichibu Cement K K) which discloses a hydraulic cement which is allegedly not subject to efflorescence. The cement comprises specified relative amounts of a calcium sulphoaluminate or calcium aluminate compound, a calcium silicate compound, calcium sulphate, granulated blast furnace slag and a minor amount of an oxycarboxylic acid. The patent states that in order to ensure that no efflorescence is generated, it is very important that all the calcium hydroxide present in the system is consumed by reaction with the calcium sulphoaluminate (C4A3ŝ) and the gypsum (CŝH2) to form ettringite, (C3A.3Cŝ.32H), according to the formula:C4A3ŝ+8Cŝ+6CH+90H→C3A.3Cŝ.32H
The oxycarboxylic acid is said to reduce the generation of calcium hydroxide in the product so that this reaction can proceed to completion.
We have found however that, although the oxycarboxylic acid and the formation of ettringite may act to eliminate the precipitation of CH and therefore to reduce efflorescence, the physical properties of the product obtained from this cement are not sufficient for many applications. In particular, we have found that compositions prepared according to such a formulation suffered dimensional instability i.e. expansion in humid conditions and very high porosity. These properties were found, in comparison with ordinary mortars, to have an adverse effect upon other properties such as strength, permeability, acid resistance and a relatively high leach rate when exposed to water. Furthermore, we have found that such an open structure resulted in a risk of carbonation which would further reduce durability.